The present invention relates to an active matrix type liquid crystal display panel, an active matrix type liquid crystal display device, and a liquid crystal television.
A liquid crystal display device is composed essentially of two glass substrates that are separated to form a predetermined space into which a layer of liquid crystal is injected. A polymer thin film, called an alignment film, is disposed between the glass substrate and the liquid crystal layer, and the alignment film is processed for aligning the liquid crystal molecules. Displays are produced by such liquid crystal display devices as follows. That is, when an electric field is applied to the orderly arrangement of liquid crystal molecules, it changes the alignment direction of the molecules, which, in turn, results in a change in the optical characteristics in the liquid crystal layer.
In the conventional active drive type liquid crystal display device, a transparent electrode is disposed on an inner side of each of the pair of substrates that sandwich the liquid crystal material, and the liquid crystal molecules are driven by an electric field that is generated between these electrodes in a direction perpendicular to the substrate plane, as represented by the TN system. That is, optical switching is performed when the liquid crystal molecules stand upright in response to the electric field developed perpendicular to the substrates. In such a system, a tone of color can be inverted depending on the direction in which the viewer watches the screen of a liquid crystal panel, particularly in the case of a halftone, presenting a big problem of allowing only a narrow viewing angle range.
Japanese Patent Publication No. 63-21907 discloses an IPS system as a solution to this problem. FIGS. 32, 32A, 32B and 33 show an electrode structure in the form of a cross-sectional view of a pixel portion of a liquid crystal display device using the IPS system. The electrode structure shown in these figures represents particularly a structure having two divisions within one pixel. FIG. 34 is a schematic diagram showing a system that drives the liquid crystal display device. According to this system, two different types of electrodes, namely a pixel electrode 106 and a common electrode 103, are formed on one of a pair of substrates that sandwich the liquid crystal material, and optical switching is performed by rotating the liquid crystal molecules through a plane that is substantially parallel with a substrate plane using a uniform electric field 5 that is produced across these two electrodes and is disposed substantially in parallel with the substrate plane. This eliminates the possibility of inversion of gradation and a tone that otherwise occurs depending on the angle at which the viewer watches the screen (viewing angle), thus offering a wider viewing angle as compared with the conventional TN system.
The industry expects much of the IPS system, which has superior viewing angle characteristics, as explained in the foregoing discussion, as a new liquid crystal display device supplanting the conventional TN system, and it involves a kind of technology that is indispensable to large-scale screen liquid crystal monitors and liquid crystal televisions to be provided in the years to come. The biggest problem with the existing IPS liquid crystal display device is to determine how the response speed can be increased, considering its future applications in display devices supporting moving pictures, such as liquid crystal televisions and DVD video reproducing monitors.
Various approaches have so far been taken with respect to the need to make the IPS system liquid crystal display device faster in response in terms mainly of liquid crystal materials. Among them, efforts have been made to develop new liquid crystal materials along two major approaches. That is, the major approaches have been toward greater polarization (greater Δε) and lower viscosity of the liquid crystal material. Generally speaking, however, the greater the polarity, the higher the viscosity with liquid crystal materials. This tradeoff relationship, coupled with the fact that the characteristics of a liquid crystal material directly affect the display characteristics of, as well as the reliability in, the liquid crystal display device, imposes restrictions on the latitude allowed in material development; and, thus, there is a limit to achievement of a higher speed response merely from the approach of material development. In addition to the effort made to develop such liquid crystal materials, an approach is also being taken toward achieving higher response speeds from the viewpoints of driving method and pixel structure.
Japanese Patent Laid-open No. 2001-34238 discloses a new approach toward higher response speed by means of an improved driving method. The method is concerned with an approach, in which display data exceeding variations in gradation (an overdrive voltage) is applied to the pixel to augment the amount of voltage change, thereby increasing the response speed. Generally called overdrive driving, the method reduces the time it takes the device to reach a target transmittance (response time) by temporarily applying, immediately after there is a change in gradation, a voltage higher than that required for achieving the target transmittance. This method is effective only in the case of a halftone, which will be described later.
Japanese Patent Laid-open Nos. 11-231344 and 11-316383 disclose means for realizing a higher response speed by changing the pixel structure. What is disclosed in Japanese Patent Laid-open No. 11-231344 is a structure in which, in the conventional IPS system liquid crystal display device, a second common electrode is formed on the substrate opposite the array substrate (electrode substrate), on which a pixel electrode and a common electrode are formed. This structure is concerned with a method in which an electric field generated across the pixel electrode and the second common electrode is used to effectively drive liquid crystal molecules near the opposing substrate, thereby increasing the response speed. Japanese Patent Laid-open No. 11-316383 discloses a method for increasing the response speed in which a structure is employed, wherein the electrode spacing between the pixel electrode and the common electrode is made narrower than a cell gap (liquid crystal layer thickness), or the pixel electrode is superposed over the common electrode, thereby generating a particularly strong electric field on electrode end portions.
Each of the foregoing approaches has the following problems.
Overdrive driving is a means effective only in the case of a halftone, and the biggest problem with this method is that it is unable to improve the response speed throughout all gradations. The voltage applied to the liquid crystal layer of the liquid crystal display device is usually an ac voltage, and there is no such thing as a voltage that is lower than no applied voltage (V=0V) on the low voltage side. This means that the method is unable to increase the response speed, with which gradations are rendered by the no voltage condition. Moreover, on the high voltage side, too, the voltage to be applied to the pixel is limited because of the electrical withstand voltage of the signal line driver, and it is generally equivalent to one that sufficiently drives the liquid crystal (operating voltage). As a result, since there is substantially no voltage range available for use in making the response speed faster on the high voltage side (ΔV: overdrive voltage), the method is unable to make the response speed faster, with which gradations are rendered by the high voltage side. That is, overdrive driving can increase the response speed in the case of a halftone and flattens out the dependency of the response speed on gradations, but it is unable to improve the response speed particularly on the high voltage side because of the restrictions imposed by the withstand voltage of the driver.
High-speed response technology by means of pixel structure has a problem of degraded display performance. According to the technique disclosed in Japanese Patent Laid-open No. 11-231344, in which the second common electrode is formed on the opposing substrate, the perpendicular electric field component increases, in addition to a horizontal electric field component, which causes the liquid crystal molecules having a property of positive permittivity anisotropy to stand up with respect to the substrate plane, thus presenting a problem in color reproduction.
In the structure disclosed in Japanese Patent Laid-open No. 11-316383, in which the electrode spacing is made narrower than the cell gap (liquid crystal layer thickness), an image sticking phenomenon that is counted among the factors seriously degrading display performance occurs. As found through examinations made by the inventors, a strong electric field that is produced on the electrode end portions, and that functions effectively for increasing the response speed, is the major reason contributing to this image sticking phenomenon. The liquid crystal molecules twist through a plane parallel with the substrate plane according to the electric field strength. If the electrode spacing is extremely narrow due to the electrodes being superposed one on top of another, however, the strong electric field generated as a result of electric field concentration on the electrode end portion causes the liquid crystal molecules in the liquid crystal layer to twist about a twisting angle several-fold greater than the average. To produce a white display, for example, the average longer axes of liquid crystal molecules in the liquid crystal layer must be twisted by about 45° with respect to the transmission axis of a polarizer. There is a distribution of this twisting angle of the liquid crystal molecules; and, in areas near the electrode end portions where the electric field is extremely strong, the molecules are twisted by more than 45°. Particularly in areas near the electrode end portions, a torque generated by this excessively great twist of liquid crystal molecules is transmitted at this time as a load on the surface of the alignment film; and, as a result, the image sticking or seizure phenomenon probably occurs mainly due to plastic deformation of the alignment film surface. This image sticking phenomenon has a strong correlation with the period of time through which the strong electric field is held. The longer the retention period, the more the phenomenon is aggravated.